1. Field of the Invention
The present invention relates generally to light emitting devices and more particularly to light emitting diodes with enhanced light extraction efficiency.
2. Description of the Related Art
The light extraction efficiency of a light emitting diode (LED) is defined as the ratio of the LED's external quantum efficiency to the LED's internal quantum efficiency. Typically, the light extraction efficiency of a packaged LED is substantially less than one, i.e., much of the light generated in the LED's active region never reaches the external environment.
Light extraction efficiency is reduced by total internal reflection at interfaces between the LED and surrounding material followed by reabsorption of the totally internally reflected light in the LED. For example, for a cubic geometry LED on a transparent substrate encapsulated in epoxy, the refractive index (n) at the emission wavelength changes from a value of, for example, nsemi ˜3.5 in the LED semiconductor to nepoxy ˜1.5 in the epoxy. The corresponding critical angle for total internal reflection of light incident on the epoxy encapsulant from the LED semiconductor of this example is θC=arcsin(nepoxy/nsemi)˜25°. Neglecting scattering and multiple reflections, light emitted over 4π steradians from a point in the active region of the cubic LED crosses a semiconductor/epoxy encapsulant interface only if it is emitted into one of six narrow light cones, one for each interface, with each light cone having a half angle equal to the critical angle. Additional losses due to total internal reflection can occur at the epoxy/air interface. Consequently, an efficient conventional geometry (for example, rectangular parallelepiped) transparent substrate AlInGaP LED encapsulated in epoxy, for example, may have an external quantum efficiency of only ˜40%, despite having an internal quantum efficiency of nearly 100%.
The effect of total internal reflection on the light extraction efficiency of LEDs is further discussed in U.S. Pat. No. 5,779,924; 5,793,062; and 6,015,719 incorporated herein by reference.
In one approach to improving light extraction efficiency, LEDs are ground into hemispherical shapes. Light emitted from a point in the active region of a hemispherically shaped LED intersects the hemispherical interface at near normal incidence. Thus, total internal reflection is reduced. However, this technique is tedious and wasteful of material. In addition, defects introduced during the grinding process may compromise the reliability and performance of the LEDs.
In another approach, LEDs are encapsulated (encased) in a material with a dome or hemispherically shaped surface. For example, the epoxy encapsulant of the above example may be dome shaped to reduce losses due to total internal reflection at the epoxy encapsulant/air interface. However, shaping the surface of a low refractive index encapsulant such as epoxy does not reduce losses due to total internal reflection at the semiconductor/low index encapsulant interface. Moreover, epoxy encapsulants typically have coefficients of thermal expansion that poorly match those of the semiconductor materials in the LED. Consequently, the epoxy encapsulant subjects the LED to mechanical stress upon heating or cooling and may damage the LED. LEDs are also encapsulated in dome shaped high index glasses, which increase the critical angle for the semiconductor/encapsulant interface. Unfortunately, optical absorption in high index glasses and mechanical stress typically degrade the performance of an LED encapsulated in such glass.
What is needed is a method for increasing the light extraction efficiency of light emitting diodes which does not suffer from the drawbacks of previous methods.